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Top balancing, recommendations (fastest or best practices)

I think individual top balancing is more time consuming and adds more chance for error.
More chance for error on one cell. There are people around here that have messed up 16 cells at once. Some people did that multiple times.
I will start with one cell. Then move to 4. It will probably be months until I am comfortable enough to set up a 48v battery pack.
I will probably reduce the state of charge of charge of cells that will be laying around or if I need to remove them from my anti bloat restraining fixture.
These discussions give me a lot to think about.
 
But if it's parallel balancing, the cell with the highest voltage (and likely SoC) will discharge to all the other cells, therefore bringing its SoC down, so it cannot stay at 3.65 while the rest are not.
 
Hey all... First post...

I've got 16 X 300Ah cells pulled from a 12v battery module, the BMS wasnt doing much of a job in balancing, so I stripped the cells out and removed the BMS. In each battery module one cell was tripping HVC.

My question is:
When I parallel the cells to balance, what is the maximum differential voltage to avoid excess current? Currently they're all between 3.35V ish and 3.65V ish.
I was planning on using a load resistor to pull down the high cells to withing 100mV of each other then connecting them all together and charging them to 3.6V with a benchtop supply.

From there I've got a Batrium BMS coming (expensive but good), and then I'll be able to connect it all up to the Sungrow SH5k inverter.

That's the plan ;)

Cheers,
Andy
 
It is unusual for a LFP cell to rest at 3.65 volts. Can you confirm the chemistry of those cells? If these are another chemistry then thy are at low SOC and you are not going to see a welding are when you put them in parallel. You could measure the Amps if you put the load resistor between two cells in parallel. The resistor will reduce any excess current howevrr excess is defined.
Beyond this simple question you may want to start your own thread so you can get more focused responses that will be easier for you to follow.

Welcome to the forum.
 
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They're LiFePO4.

3.65 is not the resting voltage. All 4 batteries were charged before removing the cells from the casing, but in each battery there was a cell that reached HVC before the whole battery got to full charge.

The other cells would have a pretty high SOC I imagine.

It's all good, I'll match those cells that are close together and deal with the ones that are >0.1V individually before connecting them.

Cheers
Andy
 
Here's my thoughts, they are worth exactly what you paid for them. I am a proponent of single cell balancing (and load testing) just because it requires more time and user interaction. I also document each cells performance manually, which takes even more time. I never turn my back on a cell until I have documented it's characteristics over time. I use manufacturer specs, nothing else, to judge the results. Cells are electrochemical devices that have known characteristics which makes them predictable, but those characteristics will be slightly different even when new because nothing is perfect. Over time those characteristics will become evident. Time spent with your cells is a good thing.

I use a fixed rate CC/CV charger that is not affected by finicky adjustments because mine is likely to be bumped. Once the cell hits 3.65v, the charge is terminated. The constant voltage portion of the charger is never used.

These thoughts are based on my time in the Pb industry. I'm an old dog that doesn't like change and has forgot much more than I remember (it's getting worse), but LFP is a new chemistry to me and I'm building my last system so I will gladly listed and learn.

Mick Jager sang it best, ?Time is on your side.?

Practice Slow Water.
Rob
 
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I use a fixed rate CC/CV charger that is not affected by finicky adjustments because mine is likely to be bumped. Once the cell hits 3.65v, the charge is terminated. The constant voltage portion of the charger is never used.

That should work fine as long as you're charging at the tail current, i.e., a 100Ah cell should hit 3.65 at about 5A. If you're charging at higher than 5A, you're terminating early and leaving capacity on the table, possibly a lot depending on how close you are to 0.5C.
 
That should work fine as long as you're charging at the tail current, i.e., a 100Ah cell should hit 3.65 at about 5A. If you're charging at higher than 5A, you're terminating early and leaving capacity on the table, possibly a lot depending on how close you are to 0.5C.
Thank you for the input. I really appreciate all the input from this forum. Section 5 of the EVE LF280 spec sheet doesn't address balance charging so I'm glad I didn't try a .5C balance charge.:) The balance charger I used is 10 amps and it held 9.77 amps for the entire charge to 3.656 when the current started tapering quickly. It was only a minuet or two @ 3.656. Some of the cells took as little as 10 minuets and some took as long as 76 minuets. Hopefully the short charge period only means they shipped near fully charged.
 
I don't believe you have to step balance (like some say - 3.4, then 3.5, then 3.6/3.65).

I'm just doing it so I can physically be there to watch it go from 3.55 to 3.6 (just personal satisfaction, ha ha).

Max current of my benchtop DC power supply is, in the manual, 20A. Just using 16-17A bec not sure if these inexpensive Chinese power supplies can do 20A continuous. So, derating it about 20%.
So how did it go? How many days did it stay at the same voltage. I've got 16 100AH Cells in parallel right now and I'm wayyyyyyyy to impatient for this. LOL
 
So how did it go? How many days did it stay at the same voltage. I've got 16 100AH Cells in parallel right now and I'm wayyyyyyyy to impatient for this. LOL
Took about 10-11 days. Hit 3.5v and stayed there (when I saw it, current was very low, like 2 amps). Put it to 3.6v, and within 2 hours if I remember correctly, it hit 3.6v.

Disconnected the power supply, and after 30 minutes, it was still about 3.55v, and after about an hour, it was still 3.5. So I called it good (I read somewhere that if it stays over 3.5v for 30 minutes, it's pretty much full to the brim).

It's operational now, and handling my day-to-day battery needs, and, in fact, just realized that I do not have enough panels for my household loads AND charging the battery (I have about 7.8kW of panels, but have many kids, and online school and work from home means higher consumption, so not a lot of wattage left to give the batteries a good charge). There was a point where I "channeled" all the power into charging (rather than supplying daytime loads), and I got it fully charged.
 
I charged 8 of the 280 Ah cells in parallel with my benchtop power supply unit in less than 24 hours. This was after letting them rest in parallel for a few hours.

I don't know if it's that my PSU (30v, 10a) is more powerful (doubtful) or the cells were already at a good voltage level, but the charge took much less time than I expected.
Wow, Did you get all those 8 cells up to 3.6 or 3.65 volts? Just wondering while ... I am having very slow charging cycle on one 280 Ah cell with one of those $56 30v 10a Bench Power Supplies on Amazon. Wondering if maybe I need a better power supply, or maybe have a subborn below par battery. Currently looking at a 3.65v 20 amp charger on Alibaba that looks like an attractive good bet: https://www.alibaba.com/product-detail/3-65V-20A-Single-Cell-Lithium_60830518866.html ... got quote for $81 with DHL shipping to USA. (still looking for more specifics on specs) :+)
 
Wow, Did you get all those 8 cells up to 3.6 or 3.65 volts? Just wondering while ... I am having very slow charging cycle on one 280 Ah cell with one of those $56 30v 10a Bench Power Supplies on Amazon. Wondering if maybe I need a better power supply, or maybe have a subborn below par battery. Currently looking at a 3.65v 20 amp charger on Alibaba that looks like an attractive good bet: https://www.alibaba.com/product-detail/3-65V-20A-Single-Cell-Lithium_60830518866.html ... got quote for $81 with DHL shipping to USA. (still looking for more specifics on specs) :+)

Take a picture of the front of your power supply so we can see your settings.

I went to 3.65.
 
Take a picture of the front of your power supply so we can see your settings.

I went to 3.65.
I got my CV set at 3.65 volts with the green light on next to volt knob, and CC current shows up at 5.89 Amps ... My one 280Ah cell is slowly going up; now it's at 3.349 V at the battery terminals. This was a problem battery that was short on Ahs, I am wanting to fully charge and test with my 20 amp draw down fan unit for Ah. ... Maybe this particular battery is my issue. Just hard to image 10 amps on 8 x 280 Ah cells in parallel. When I go for my top balance, I will charge my 24 v battery bank in series to near full, then go for parallel balancing; and see if I can hit 3.65V. Your story is inspiring. I did the parallel sit and balance a few months ago, and now replacing one cell that drained down early to 2.5v while the other 7 cells had about 2.85v . So since this is the cell I am trying to get up to 3.65v ... this particular cell might be the biggest factor in my slowish charging cycle. One my old pictures from BMS info showed an internal resistance on this cell of 2.9 ohms when in use at 95A while the 7 other cells had 0.6 to 0.8 ohms internal resistance. Maybe that why it seem to be taking so long to charge. (seems to make better progress turn Voltage Up for max CC 10A) ... I think I will move my attention to my full battery bank that need a physical top balance with a newly inserted replacement cell I want to confirm has it 280 Ahs. 10 amps for parallel charging 8 x 280Ah to 3.65V seems low to me; so your story is inspiring. :+)
 
I plan to series-then-parallel the 280's I just got. Final build is 2 48v packs so I'll do 2 separate series-then-parallel charges (keeping cells 1-16 and 17-32 assigned to their own packs). I have a benchtop PS that I can set to 3.65v (10 amps) and will monitor the cells manually. I just did this with 100's and that went well.
 
Take a picture of the front of your power supply so we can see your settings.

I went to 3.65.
I got my top balancing act together for 8 c 3.2v 280 Ah LiFePO4s up to 3.65V. After getting up to 3.4, it kept going up faster than I thought it would. I was discouraged earlier trying to charge the defective old LiFePO4 I just replaced, ... to test it for Ah and more hands on lessons. That particular below par battery would not charge up past 3.36V on my bench power supply even at CC for some reason (was wondering if my power supply was defective). I have same defective battery cell on a 20 amp draw Ah Test as I write this. ... RE full 3.65V top balancing; My 30V . 10A specs Bench Power Supply was good enough to get my top balancing up to 3.65V on paralleled 8 cells after I charged the full set in series as best I could before hand. ... I will next be Testing My Newly Replaced Battery cell that Xuba sent me as a replacement as it now sits fully charged and balanced within my full set; Testing via various amp draw down cycles (like 5 amp to 100 amps) by running my house on just these batteries alone to set what they can do without any solar input . On that Test, I will be getting info from my previously calibrated and set up Chargery BMS8T to look at details like cell voltage differences, and Ah and Amp In or Out values etc. Plus be comparing that to info. I also from my three LV2424 All In One units (digital interface/ pages of info.) connected to my battery 24V bank. Hope it all looks good including with this new Replacement LifePO4 cell that previously looked defective to me; Before I got my remeinder and clues about how important Top Balancing Is. I had proceeded with my previous top balance of original set, and while trying to be close in voltage, I then just swap in my new replacement battery; THAT was Not Good Enough, Nor a worthy manuever. ... My Testing of New Cell is likely coming later today into tomorrow. :+)
 
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Yes I would definitely series charge them or it may take forever to top balance them in parallel. Even if half full it will take more than 10 days if you set your power supply to 3.65 as the amps will decrease rapidly as voltage increases. Im willing to bet it will take about 14 days.
I agree with the idea of series charging close to full charge first, unless your close. Re: Top Balancing in parallel: I thought this was an interesting option: a 3.65v 20 amp charger on Alibaba : https://www.alibaba.com/product-detail/3-65V-20A-Single-Cell-Lithium_60830518866.html ... I recently got quote for $81 with DHL Air shipping to USA. (was still looking for more specifics on specs) :+) ... for my 8 x 3.2V 280Ah. I aborted that possible purchase when I out found my up to 30V x 10A spec Bench Power Supply worked faster than I thought it would after being near full battery voltage on a a series charge. I had one replacement battery I tried to slip in at close to matched voltage, and that was a big lesson for me on how important top balancing is. ... I would likely purchase that 20 Amp charger if I was Top Balancing a 48v 280 Ah set. or had multiple sets to work on, ... and test it out.
 
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